Continuous galvanizing furnaces according to the current state of the art are conventionally composed of several zones through which the strip passes in succession:                in certain cases, a bare-flame preheating zone, that is to say one provided with burners which develop their flame directly in the chamber. This zone conventionally allows the strip to be raised from room temperature to a temperature of about 650 to 700° C.;        a radiant-tube heating zone in which the temperature of the strip is raised to about 700-900° C. This heating zone is placed in a reducing atmosphere so as to make it possible to reduce the oxides formed at the surface of the strip, particularly iron oxides, and in all cases not to create any oxide if there had been none previously;        a holding zone in which the strip remains at a hold temperature for a time defined by the type of thermal cycle to be produced; and        one or more cooling zones with controlled cooling rates depending on the type of thermal cycle to be produced. This cooling is carried out until a temperature close to that of the zinc bath, typically 460° C., is achieved.        
It has been found that in the galvanizing furnaces according to the prior art, the strip runs through a reducing atmosphere from the inlet right to the outlet of the furnace or, if a bare-flame preheating zone exists, from the outlet of the latter to the outlet of the furnace. The reducing atmosphere is therefore maintained in the furnace at the latest after the outlet of the preheat, i.e. conventionally at a strip temperature of 650 to 700° C. The object of this process is to limit the formation of oxides, mainly iron oxides, on the surface of the strip and to reduce them if any exist or if any is formed in the preheat, so as to allow good bonding of the zinc to the surface of the strip in order to obtain a high-quality galvanized product.
The residence of the strip in this reducing atmosphere must take place under sufficient conditions (temperature, residence time and dew point of the atmosphere in the furnace) in order for the strip to undergo cleaning therein compatible with good quality of the subsequent coating, in particular good quality of zinc adhesion.
Current developments in steels aimed at increasing their mechanical strength result in an increase in the content of alloying elements such as Si, Cr, Mn, etc.
It should be pointed out these new addition elements form oxides that are more stable than iron oxides contained in the structure of the strip. These elements are therefore hungry for oxygen, thereby causing them firstly to be oxidized on the surface of the strip where oxygen is present, even in a low concentration. Since these oxides have consumed the Si, Cr and similar atoms available on the surface, these elements are present in lower concentration thereon. To compensate for this decrease in concentration, the neighbouring Si, Cr or similar atoms will therefore migrate by diffusion from the interior towards the surface, thereby feeding the oxidation reaction. This migration is thermally activated, that is to say accelerated by time and above all by temperature. Consequently, it does not take place in the bare-flame preheat section, since, although the atmosphere is rich in oxygen, the strip remains therein for too short a time at high temperature because of the high heating rate. On the other hand, the diffusion of oxidizable atoms will become substantial in the heating and holding sections as the strip there is hotter, reaching its maximum temperature with longer residence times.
In the reduction section of the furnace, the iron oxides, which are more easily reducible, will be removed. The more stable Si and similar oxides will be more difficult to reduce and will remain, forming a continuous or discontinuous film which acts as an obstacle to good adhesion of the zinc coating.
Existing furnaces are therefore not suited to galvanizing the new high-strength steels because of their richness in oxidizable elements such as Si, Cr, etc. These steels must be galvanized:                either cold, that is to say electrolytically. This solution allows the desired grade to be used, but is much more expensive to implement;        or hot, but in this case it is necessary either for the steel to be rapidly (quench) cooled or for its grade to be refined.        
Quenching the steel allows the concentration of addition elements to be limited therein, but requires rapid cooling to be carried out after annealing. This cooling allows the formation of multiphase structures which provide the desired hardening properties. However, this technique is still little used.